Method and device for producing an electric insulator made from plastic

ABSTRACT

The invention relates to a method and a device for producing an electric insulator ( 1 ) made of plastic comprising a central hollow core ( 2 ) in addition to a plurality of insulating screens ( 3 ) which are arranged in a coaxial manner around the hollow core ( 2 ) and which are disposed in the axial direction of said hollow core ( 2 ). The aim of the invention is to improve the production of said plastic insulator ( 1 ) by using a method comprising, essentially, the following steps: a) placing a casting mould ( 4 ) on a predetermined axial position around the vertically arranged hollow core ( 2 ), the casting mould ( 4 ) having a coaxial opening ( 5 ) which is arranged in an axial manner in a lower section ( 6 ) of the casting mould ( 4 ), and a superior section ( 7 ) being open towards the top for forming an insulating screen ( 3 ); b) producing a tight connection ( 8 ) between the hollow core ( 2 ) and the casting mould ( 4 ) in a lover section ( 9 ) in the axial direction of the casting mould ( 4 ); c) injecting a liquid plastic material at a pressure (p e ) which is higher than the ambient pressure (p 0 ) in an axial section ( 10 ) of the casting mould ( 4 ), which is arranged axially above the tight connection ( 8 ) and axially below the upper end ( 11 ) of the casting mould ( 4 ).

[0001] The invention relates to a method and a device for producing anelectric insulator which is made of plastic and has a centrally arrangedcore or hollow core, particularly made of a glass-fiber-reinforcedplastic material, as well as a plurality of insulation screens which arearranged coaxially around the core or hollow core and in an offsetmanner in the axial direction of the core or hollow core.

[0002] A method as well as a pertaining device for producing such aplastic insulator are known from German Patent Document DE-AS 2044179.There, a casting mold, which is open at the top, is arranged coaxiallyaround a centrally arranged and vertically positioned core. This castingmold is then filled by pouring in a liquid silicone mass. In this case,with respect to its cavity, the mold has a contour which corresponds tothe desired insulation screen. After the hardening of the silicone mass,the casting mold is moved axially downward, specifically by the amountwhich corresponds to the desired axial spacing of two insulationscreens. The individual insulation screens are cast in this manner.

[0003] It was recognized to be a disadvantage of the known method thatthe silicone mass, from which the insulation screens are cast, shouldhave a relatively low viscosity, particularly equal to or lower than25,000 [mPa*s]. In order to obtain a surface of the insulation screenswhich is favorable with respect to the insulation, filling materials canbe added to the silicone mass only to a very limited degree. Thisincreases the costs of the material of the insulator considerably.

[0004] It is therefore an object of the invention to further develop amethod and a pertaining device of the initially mentioned type such thatit becomes possible to process also more moderately priced plasticmaterials, particularly a silicone material, with a higher viscosity andto nevertheless ensure that the cast insulation screens are of a highqualitative value. Furthermore, a shorter production time during themanufacturing of an insulator should be implemented by means of themethod according to the invention.

[0005] Thus, it is endeavored to reduce the average mold filling time by60 to 80% in comparison to known methods.

[0006] With respect to the method, this object is achieved by means ofthe invention as a result of the following steps:

[0007] a) First, the casting mold for the casting of the insulationscreens is placed in a defined axial position of the vertically arrangedcore or hollow core. In this case, the casting mold surrounds the coreor hollow core with a coaxial opening which is arranged in an axiallylower area of the casting mold; the casting mold continues to have amold area which is open at the top for forming an insulation screen.

[0008] b) Subsequently, a sealing connection is established between thecore or hollow core, on the one hand, and the casting mold, on the otherhand, in an axially lower area of the casting mold. This takes place,for example, by activating a seal.

[0009] c) Then a liquid plastic material is injected at a pressure whichis increased with respect to the ambient pressure into an axial area ofthe casting mold which is situated axially above the sealing connectionand axially below the upper end of the casting mold. This takes placeuntil the casting mold is filled with a defined amount of the plasticmaterial.

[0010] d) Subsequently, the plastic material is cured in the castingmold.

[0011] e) After the curing, the sealing connection between the core orhollow core and the casting mold is eliminated. In particular, a seal inthe casting mold is deactivated.

[0012] f) Then the casting mold is displaced relative to the core orhollow core axially downward by a defined displacement path whichcorresponds to the axial spacing of the insulation screens.

[0013] g) Steps b) to f) are repeated until the desired number ofinsulation screens has been produced.

[0014] According to a further development, the injection of the plasticmaterial takes place at an injection pressure which is at least 5 timesthe ambient pressure. The temperature of the casting mold is preferablyadjusted, particularly by heating, at least during the curing step.Particularly when silicone is used as the plastic material, this heatingtakes place to at least 60° C.

[0015] According to the invention, the device for producing the electricplastic insulator is characterized in that at least one injectionelement or injection duct for the pressure injection of liquid plasticmaterial is arranged between the lower end of the axially lower area andthe upper end of the casting mold.

[0016] The injection element or the injection duct is preferablyarranged in an axial area of the casting mold in which the core orhollow core in a coaxial opening of the casting mold rests closelyagainst the casting mold. The economic efficiency of the production canbe further improved in that several injection elements or injectionducts, preferably four, six or eight, are arranged in a uniformlydistributed manner along the circumference of the casting mold.

[0017] A seal for sealing off the casting mold with respect to the coreor hollow core is preferably arranged below the axial position of theinjection elements or injection ducts. The seal can advantageously beactivated and deactivated respectively in a controllable manner,particularly a pneumatic or hydraulic operation being considered here.Finally, temperature-adjusting elements, particularly heating elements,can be arranged in the mold area of the casting mold which is open atthe top.

[0018] The core idea of the novel method has the object that themanufacturing of an insulation screen by means of a casting mold takesplace such that the plastic material, particularly the siliconematerial, is fed at an increased pressure from below into the castingmold which is open at the top. In contrast to the known method, theliquid plastic material is therefore no longer poured at an ambientpressure into the open casting mold from the above.

[0019] This results in important advantages:

[0020] First, a moderately priced starting material can be used. Alarger quantity of fillers can be added to the casting or siliconematerial, which lowers the total costs of the material.

[0021] Furthermore, liquid plastic material of a high viscosity can beprocessed without reducing the quality of the surface of the insulationscreens. As a result of the pressure casting, it becomes possible toprocess plastic masses whose viscosity is clearly about 25,000 [mPa*s],at least up to 85,000 [mPa*s]. So far, the used casting or siliconematerial has had to be very liquid in order to be able to producehigh-quality insulation screens.

[0022] This also makes it possible to use a casting or silicone materialwhich has a shorter curing time, which reduces the production time for aplastic insulator and correspondingly increases the economic efficiencyof the method.

[0023] By means of the suggested method, seamless insulation screenswithout any bur can be produced, in which case the casting or siliconematerial is applied directly to the central core.

[0024] The drawing shows an embodiment of the invention.

[0025]FIG. 1 is a schematic sectional view of a plastic insulator;

[0026]FIG. 2 is a sectional view of a casting device whose seal is notactivated;

[0027]FIG. 3 is the same view as that of FIG. 2 but with an activatedseal.

[0028]FIG. 1 is a view of an electric plastic insulator 1. It consistsof a centrally arranged hollow core 2 around which insulation screens 3are arranged in a coaxial manner. In this case, the screens 3 are eacharranged at a constant mutual spacing s which corresponds to the axialdisplacement path of the casting mold 4 described in the following.

[0029]FIG. 2 again shows the centrally arranged hollow core 2 of theplastic insulator 1 whose axis is positioned vertically. The castingmold 4 is used for producing the individual insulation screens 3. Thecasting mold has a coaxial opening 5 which reaches around the hollowcore 2.

[0030] The casting mold 4 has an axially lower area 6 which is placedessentially with the coaxial opening 5 of the casting mold 4 closelyaround the hollow core 2. A mold area 7 of the casting mold 4, which isopen at the top, extends axially above the lower area 6. This mold area7 has the actual cavity of the casting mold 4 which defines the shape ofthe insulation screen 3 to be cast.

[0031] When the hollow core 2 is arranged in a vertical manner, thecasting mold 4 is moved downward by means of moving elements which arenot shown (see double arrow P in FIG. 3). After an insulation screen 3has been manufactured, the casting old 4 is moved by the axialdisplacement path s illustrated in FIG. 1 in the downward direction.During the movement,—as illustrated in FIG. 2—, the seal 13 isdeactivated which is provided in order to establish a fixed sealingconnection 8 (see FIG. 3) between the casting mold 4 and the hollow core2.

[0032] When the casting mold 4 has reached the desired axial position,it is applied to the hollow core 2 preferably by the buildup of ahydraulic or pneumatic pressure in the seal 13. As a result, the sealingconnection 8 is established; see FIG. 3. In this case, the sealingconnection 8 extends along the axial area 9 which is illustrated inFIGS. 2 and 3.

[0033] Directly axially above the area 9 of the sealing connection 8, aninjection element or injection duct 12 is arranged in the axial area 10.This area 10 is still situated in that axial area of the casting old 4which is essentially placed closely around the hollow core 2. By way ofthe injection element or the injection duct 12, the liquid siliconemass, preferably with fillers, is injected into the casting mold 4. Inthis case, the mass may have a viscosity of at least up to 85,000[mPa*s]; thus, a value which is considerably above the viscosity valueof 25,000 [mPa*s] which has so far been considered to be the limitvalue. As a result, so-called LSR silicones (liquid silicone rubber) canbe used which has not been possible by means of the previously knownmethod.

[0034] In this case, the injection of the silicone material takes placeat an injection pressure P_(e) which is considerably above the ambientpressure P₀. The injection pressure is at least 5 times the ambientpressure.

[0035] It is essential to the invention that the area of the injectionpoint(s) 10, that is, the axial point at which the injection element orthe injection duct 12 is arranged, is situated between the lower end 15of the lower area 6 of the casting mold 4 and its upper end 11.

[0036] For accelerating the curing process of the silicone material,heating elements 14 are arranged in the casting mold 4 in the proximityof the cavity wall, which heating elements 14 maintain a temperature ofat least 60° C. during the curing.

[0037] A plurality of injection elements or injection ducts 12 arearranged in an equidistant manner along the circumference of the castingmold 4, which is not shown in FIGS. 1 to 3. Preferably four, six oreight injection elements or injection ducts 12 are used. The individualinjection elements or injection ducts 12 may be supplied with siliconematerial by a coaxially arranged ring conduit.

[0038] A rotation of the casting mold, as occasionally known from theprior art, is not necessary in the case of the method according to theinvention. Because of the elimination of the mold rotation, the siliconecan be fed without any problem by the injection elements or injectionducts 12. The electric heating of the casting mold 4 can also be easilyimplemented. As a result of the seal 13, which can be activated anddeactivated, the casting mold 4 can be axially moved without any problembecause the seal 13 is not activated during this movement.

[0039] The feeding path for the silicone from its source to theinjection element(s) or injection duct(s) 12 may be cooled in order toprevent a vulcanization there.

[0040] One or more additional temperature element(s) 16 may be arrangedabove the casting mold 4. Such temperature element(s) may particularlybe an infrared lamp or lamps.

[0041] Instead of the above-described insulation screens 4 made ofsilicone plastics, these—as generally known—may also consist of other,preferably weather-resistant synthetic resins.

List of Reference Numbers

[0042]1 Electric plastic insulator

[0043]2 core or hollow core

[0044]3 insulation screens

[0045]4 casting mold

[0046]5 coaxial opening of the casting mold

[0047]6 axially lower area of the coasting mold

[0048]7 mold area of the casting mold open at the top

[0049]8 sealing connection

[0050]9 area of the sealing connection

[0051]10 area of the injection point(s)

[0052]11 upper end of the casting mold

[0053]12 injection duct

[0054]13 seal

[0055]14 temperature-adjusting elements

[0056]15 lower end of the casting mold

[0057]16 additional temperature element)s)

[0058] P double arrow

[0059] p₀ ambient pressure

[0060] P_(e) injection pressure

[0061] s axial displacement path

1. Method for producing an electric insulator (1) which is made ofplastic and has a centrally arranged core or hollow core (2),particularly made of a glass-fiber-reinforced plastic material, as wellas a plurality of insulation screens (3) which are arranged coaxiallyaround the core or hollow core (2) and in an offset manner in the axialdirection of the core or hollow core (2), characterized by a)positioning a casting mold (4) in a defined axial position around thevertically arranged core or hollow core (2), the casting mold (4) havinga coaxial opening (5), which is arranged in an axially lower area (6) ofthe casting mold (4), as well as a mold area (7) open at the top forconstructing an insulation screen (3), b) producing a sealing connection(8) between the core or hollow core (2) and the casting mold (4) in anaxially lower area (9) of the casting mold (4), c) injecting a liquidplastic material, particularly a silicone material, at a pressure P_(e)increased in comparison to the ambient pressure (P₀) in an axial area(10) of the casting mold (4), which is situated axially above thesealing connection (8) and axially below the upper end (11) of thecasting mold (4), until the casting mold (4) is filled with a definedamount of the plastic material, d) letting the plastic material be curedin the casting mold (4), e) eliminating the sealing connection (8)between the core or hollow core (2) and the casting mold, f) axiallydisplacing the casting mold (4) relative to the core or hollow core (2)downward by a defined displacement path (s), g) repeating steps b) to f)until the desired number of insulation screens (3) have been produced.2. Method according to claim 1, characterized in that the injection ofthe plastic material takes place according to Step (c) of claim 1 at aninjection pressure (P_(e)) which is at least 5 times the ambientpressure (P₀).
 3. Method according to claim 1 or 2, characterized inthat the casting mold (4) is adjusted in its temperature, particularlyheated up, at least during Step d) according to claim
 1. 4. Methodaccording to claim 3, characterized in that, when silicone is used asthe plastic material, the casting mold (4) is heated to at least 60° C.5. Device for producing an electric plastic insulator (1), particularlyaccording to the method of one or more of claims 1 to 4 which has acasting mold (4) for producing insulation screens (3), which castingmold (4) can be arranged concentrically around a vertically positionedcore or hollow core (2) and has an axially lower area (6), which isplaced around the core or hollow core (2), as well as an axially upperarea (7) which radially widens in relationship to the axially lower area(6) and is adapted to the shape to the insulation screen (3) to beproduced, characterized in that, between the lower end (15) of theaxially lower area (6) and the upper end (11) of the casting mold (4),at least one injection duct (12) is arranged for the pressure injectionof liquid plastic material.
 6. Device according to claim 5,characterized in that the injection duct (12) is arranged in an axialarea (10) of the casting mold (4) in which the core or hollow core (2)rests closely against the casting mold (4) in a coaxial opening (5) ofthe casting mold (4).
 7. Device according to claim 5 or 6, characterizedin that several injection ducts (12), preferably four, six or eightinjection ducts (12) are arranged in a uniformly distributed manneralong the circumference of the casting mold (4).
 8. Device according toone of claims 5 to 7, characterized in that, below the axial position(10) of the injection duct or of the injections ducts (12), a seal (13)is arranged for sealing off the casting mold (4) with respect to thecore or hollow core (2).
 9. Device according to claim 8, characterizedin that the seal (13) can be activated and deactivated respectively in acontrollable manner.
 10. Device according to claim 9, characterized inthat the seal (13) can be activated pneumatically or hydraulically, inparticular, can be inflated.
 11. Device according to one of claims 5 to10, characterized in that temperature-adjusting elements (14),particularly heating elements, are arranged in the mold area (7) of thecasting mold (4) which is open at the top.
 12. Device according to oneof claims 5 to 11, characterized in that one or more additionaltemperature element(s) (16) is/are arranged above the casting mold (4).